Electrostatic Contact-Potential Mechanism for CISS Magnetoresistance

In a February 2024 preprint (arXiv:2402.00472), Sytze H. Tirion and Bart J. van Wees of the Zernike Institute for Advanced Materials at the University of Groningen propose that the large magnetoresistance (MR) observed in two-terminal experiments on chiral molecules coupled to ferromagnets does not require spin-dependent transport at all. Instead, they argue that the MR arises from an electrostatic modification of the contact potential at the chiral-material/ferromagnet interface — a mechanism that operates in equilibrium, without bias current, and that would explain the otherwise puzzling violation of the Onsager reciprocity relations in linear response.

The problem the paper addresses

The Chirality-Induced Spin Selectivity (CISS) effect is widely interpreted as evidence that chiral molecules act as spin filters: electrons of one spin orientation pass preferentially, producing a magnetoresistance when the molecule is coupled to a ferromagnetic electrode. Tirion and van Wees identify five problems with this spin-transport interpretation of two-terminal MR:

1. Unknown injection mechanism. For most chiral systems it is unclear whether the ferromagnet injects spin via the giant-magnetoresistance mechanism (spin-dependent conductivity) or the tunnel-magnetoresistance mechanism (spin-dependent tunneling density of states).

2. Conductivity mismatch. Ferromagnetic metals have far higher conductivity than the molecular systems studied. This "conductivity mismatch" problem, well known in semiconductor spintronics, is unaddressed for most CISS experiments.

3. Anomalously large MR. Reported values routinely exceed 50 % and in some experiments approach 100 %. The authors point out, following Liu and Weiss (2023), that in a standard Jullière-model analysis the total MR cannot exceed the spin polarisation of the ferromagnet, even if the chiral system were a perfect spin filter.

4. Bias independence. Theory predicts that spin polarisation should be strongly energy-dependent and therefore bias-dependent. Many experiments instead report an MR that is nearly constant over bias ranges of several volts — an "even" function of bias, whereas spin-transport theory predicts an "odd" function that changes sign when bias is reversed.

5. Onsager reciprocity failure. In the linear-response regime, the Onsager reciprocity relations require that the two-terminal resistance be unchanged when both the magnetisation and external field are reversed. Several recent experiments on chiral systems clearly violate this requirement, which Tirion and van Wees call "one of the main enigmas in the field."

The alternative explanation

The paper's central argument rests on recent experimental observations — notably by Theiler et al. (2023), Ghosh et al. (2020) and Abendroth et al. (2019) — showing that the electrostatic contact potential of chiral molecules on a ferromagnet shifts by roughly 50 mV when the magnetisation direction is reversed. These measurements, made by Kelvin probe force microscopy and photoemission, involve no electron transport current.

Tirion and van Wees argue that this equilibrium contact-potential shift modifies the Schottky-like barrier at the ferromagnet/molecule interface. Using a parallel-plate capacitor approximation, they estimate that a 50 mV shift over a typical molecular length of 1 nm requires a charge redistribution of only about 2.5 × 10⁻³ electron charges per molecule (at a packing density of 4 nm⁻²) — a small and plausible fraction. Solving the Poisson-Boltzmann equation for a doping concentration of 10¹⁵ cm⁻³ and a relative permittivity of 5, they find a screening length of approximately 150 nm, comparable to or greater than typical molecular thin-film thicknesses of less than 20 nm. The electrostatic potential landscape of the entire film is therefore modified by magnetisation reversal.

Three charge-transport mechanisms through the modified barrier are identified: thermionic emission over the barrier, quantum-mechanical tunnelling through it, and conduction within the screening region where band occupation is altered. All three depend on barrier height and shape, and hence on magnetisation direction — producing a magnetoresistance without any spin-dependent transport.

Why it resolves the Onsager problem

The Onsager reciprocity relations hold strictly in linear response and require that sample transport parameters be fully determined by equilibrium properties. In the standard CISS picture, where magnetoresistance arises from spin-dependent transmission, a spin-flip reflection mechanism (developed by Yang, van der Wal and van Wees in 2019–2021) ensures that the total charge current is unchanged by magnetisation reversal, giving strictly zero MR in linear response. Tirion and van Wees argue that their electrostatic mechanism sidesteps this constraint: because the equilibrium contact potential itself depends on magnetisation direction, the barrier and hence the linear-response resistance change when magnetisation is reversed. The Onsager relations are not violated in the usual sense; rather, the system's equilibrium state genuinely differs for opposite magnetisations.

Schottky vs. tunnel geometries

The paper distinguishes two experimental configurations. In Schottky-type junctions (direct contact between ferromagnet and chiral molecules), an increased contact potential raises the barrier height, reducing LUMO-mediated transport but enhancing HOMO-mediated transport; the sign of the MR should therefore flip when the carrier type is changed by doping or gating. In tunnel-barrier geometries (an oxide layer separates the ferromagnet from the chiral system), the same contact-potential shift that raises a Schottky barrier instead lowers the average tunnel-barrier height. Some published results reporting opposite MR signs for direct-contact and tunnel-junction measurements on the same chiral system are noted as consistent with this prediction, though the authors caution that consistent definitions of "up" and "down" magnetisation across experiments are not always clear in the literature.

The paper also discusses gold spacer layers between the ferromagnet and the chiral molecules. Ghosh et al. (2020) found that the contact-potential shift persists through 10 nm of gold and grows to over 100 mV when the gold thickness is reduced, which Tirion and van Wees take as evidence that the electrostatic modification extends well beyond the immediate interface.

Proposed experiments

Five experimental approaches are suggested to test the electrostatic mechanism:

1. Systematic comparison of contact-potential shifts with Schottky barrier heights extracted from temperature-dependent transport.
2. Changing the carrier type (electrons to holes) by doping or gating and checking whether the MR sign reverses.
3. A geometry in which the chiral material contacts a ferromagnet on one side and a non-magnetic electrode through an oxide tunnel barrier on the other — a configuration where spin-dependent transport cannot produce MR but an electrostatic contact-potential shift still can.
4. Non-local device geometries that fully decouple charge and spin transport.
5. Comparing ferromagnetic-tip-on-top vs. ferromagnetic-substrate-below configurations, which the model predicts should show opposite MR signs for the same chirality and magnetisation direction.

What the paper does not establish

The authors explicitly acknowledge that they do not identify the physical origin of the chirality-dependent contact-potential modification. They suggest a "yet unknown magnetic exchange mechanism" for direct-contact cases but note that the effect persists through 10 nm of gold, where an exchange mechanism is difficult to justify. The bias independence of the MR at voltages up to several volts is also left unexplained: at large bias both Schottky and tunnel barriers are reshaped by the applied field, and why the MR remains roughly constant is described as "a question that remains to be answered." The paper does not claim that spin-dependent transport in chiral systems is impossible — only that it cannot account for the linear-response MR and that an electrostatic mechanism provides a consistent alternative for two-terminal geometries.

Context within the CISS debate

The paper sits within a broader tension in the CISS field. The standard interpretation, associated with Ron Naaman and collaborators, treats the large MR as evidence of strong spin filtering by chiral molecules. Tirion and van Wees, working in the same Groningen group that developed the Yang–van der Wal–van Wees spin-transmission model (2019), now argue that the very model they helped build demonstrates the impossibility of linear-response MR from spin transport alone. Their alternative shifts the explanatory burden from the chiral molecule's electronic structure to the electrostatics of the molecule–ferromagnet interface — a domain more amenable to direct experimental probing but one that introduces its own unexplained ingredient: why chirality and magnetisation jointly modify the contact potential in equilibrium.

References

• Tirion, S. H. and van Wees, B. J. "A mechanism for electrostatically generated magnetoresistance in chiral systems without spin-dependent transport." arXiv:2402.00472v1, 1 February 2024.
• Yang, X.; van der Wal, C. H.; van Wees, B. J. "Spin-dependent electron transmission model for chiral molecules in mesoscopic devices." Physical Review B 99, 024418 (2019).
• Theiler, P. M.; Ritz, C.; Hofmann, R.; Stemmer, A. "Detection of a Chirality-Induced Spin Selective Quantum Capacitance in α-Helical Peptides." Nano Letters 23, 8280 (2023).
• Ghosh, S. et al. "Effect of Chiral Molecules on the Electron's Spin Wavefunction at Interfaces." Journal of Physical Chemistry Letters 11, 1550 (2020).
• Liu, T. and Weiss, P. S. "Spin Polarization in Transport Studies of Chirality-Induced Spin Selectivity." ACS Nano 17, 19502 (2023).

Further reading

• Real-Space Imaging of Spin Polarisation in CISS Devices — A subsequent experiment by Lee, Namgung et al. using RMCD on chiral tellurium nanowires with graphene electrodes provides the first direct spatial map of spin polarisation in a CISS device. The observed identical spin signs in nanowire and electrodes support the spin-polariser mechanism, whose theoretical basis also underpins several of the objections raised by Tirion and van Wees against the spin-filter model.
• The Spinterface Mechanism for CISS: A Critical Perspective — A 2025 critical perspective evaluating the spinterface model as a unified CISS explanation. The spinterface approach addresses the same anomalously large MR and Onsager puzzles that motivate the Tirion–van Wees electrostatic mechanism, but through a different route: a feedback loop between electron current in the chiral molecule and fluctuating surface magnetic moments at the electrode.
• Theory of CISS: Progress and Challenges (Evers, Kronik et al., 2021) — The 2021 community theory review, which sets out the Onsager-based ban on linear-response spin filtering in two-terminal devices and discusses the Yang–van der Wal–van Wees transfer-matrix model that this paper builds on.